In electrophotography, an electrophotographic substrate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging a surface of the substrate. The substrate is then exposed to a pattern of activating electromagnetic radiation, such as, for example light. The light or other electromagnetic radiation selectively dissipates the charge in illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in non-illuminated areas of the photoconductive insulating layer. This electrostatic latent image is then developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image is then transferred from the electrophotographic substrate to a necessary member, such as, for example, an intermediate-transfer member or a print substrate, such as paper. This image-developing process can be repeated as many times as necessary with reusable photoconductive insulating layers.
Image-forming apparatus such as copiers, printers and facsimiles, including electrophotographic systems for charging, exposure, development, transfer, etc., using electrophotographic photoreceptors have been widely employed. In such image-forming apparatus, there are ever-increasing demands for improving the speed of the image-forming processes, improving image quality, miniaturizing and prolonging the life of the apparatus, reducing production and ruling costs, etc. Further, with recent advances in computers and communication technology, digital systems and color-image output systems have been applied also to image-forming apparatuses.
Electrophotographic imaging members or photoreceptors are well known. Photoreceptors having either a flexible belt or a rigid drum configuration art commonly used in electrophotographic processes. Photoreceptors may comprise a photoconductive layer including a single layer or composite layers. These photoreceptors take many different forms. For example, layered photo-responsive imaging members are known in the art. U.S. Pat. No. 4,265,990 to Stolka et al., which is incorporated herein by reference in its entirety, describes a layered photoreceptor having separate photo-generating and charge-transport layers. The photo-generating layer disclosed in the 990 patent is capable of photo-generating holes and injecting the photo-generated holes into the charge-transport layer. Thus, in the photoreceptors of the 990 patent, the photo-generating material generates electrons and holes when subjected to light.
More advanced photoconductive photoreceptors containing highly specialized component layers are also known. For example, multi-layered photoreceptors may include one or more of a substrate, an undercoating layer, an intermediate layer, an optional hole- or charge-blocking layer, a charge-generating layer (including a photo-generating material in a binder) over an undercoating layer and/or a blocking layer, and a charge-transport layer (including a charge-transport material in a binder). Additional layers, such as one or more overcoat layer or layers, may be included as well. In view of such a background, improvement in electrophotographic properties and durability, miniaturization, reduction in cost, etc., in photoreceptors have been studied, and photoreceptors using various materials have been proposed.
As discussed above, silicon-containing compounds used in photoreceptor layers, such as in photosensitive and protective layers, have been shown to increase the mechanical lifetime of photoreceptors, under charging conditions and scorotron-charging conditions. However, there are shortcomings associated with photoreceptor layers that include silicon-containing compounds, including cross-linked siloxane-containing overcoat layers. For example, sol-gel solutions for making such layers must be prepared at the site at which the layers are prepared, because polymerization of the sol-gel solutions must be carried out during layer formation.
Sol-gel processes are generally known in the art, and embodiments of this disclosure include apparatuses and methods for preparing sol-gel solutions. In exemplary known processes, compositions formed by sol-gel processes and solutions comprise an organic-inorganic composite structure, generally characterized as an inorganic glassy polymer having an organic material dispersed in or interpenetrated into and/or chemically bonded into the inorganic polymer network.
The organic-inorganic composite structure includes a glassy polymer, such as an inorganic silica polymer such as a silica glass structure. The glassy polymer is prepared by a solution-gelation (or “sol-gel”) process during which hydrolysis, followed by condensation polymerization, of a silicon alkoxide takes place in the presence of water and an alcohol. The general process for forming sol-gels is taught, for example, in C. J. Brinker and G. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing (Academic Press, Boston, 1990). This two-step reaction process, which transforms a miscible one-phase liquid solution into a two-phase material, is called “sol-gel transition”. Generally, the silicon alkoxide/water/alcohol mixture is slow to hydrolyze. The hydrolysis rate is a function of the solution pH and, therefore, may be controlled by the addition of an acid or base as a catalyst. The reaction mixture can further include other materials, such as organic monomers or polymers or other additives, which can become either chemically bound into the glassy polymer network, or entrapped in the glassy polymer structure.
As is known in the sol-gel art, solution pH may influence the properties of the formed glassy polymer gel. Polymerization in an alkaline solution generally yields a polymer gel that is relatively porous and translucent, and is characterized by clusters of oxides of, for example, Al, B, Si, Sn, Ti, or Zr such as for example, silica, titania, alumina, zirconia, and aluminum phosphate, that are inked together to form the gel. Polymerization in an acidic solution, on the other hand, generally yields a transparent polymer gel that is characterized by a very fine porosity and by uniform, linear molecules that coalesce during subsequent sintering to form a high-density material at relatively low temperatures (e.g., about 800° C.).
Polymer gels formed by the sol-gel process are two-phase materials, denoted “alcohols,” one phase of which contains a solid siloxane skeletal network (i.e., (—Si—O—Si—)n), and an aqueous phase containing water and alcohol in the pores. Once the alcohol is formed, it is dried by slowly heating the gel to vaporize the volatile species, such as alcohol. By properly driving off the volatile species by natural evaporation, the formed polymer gel comprises a two-phase, rigid xerogel (a gel containing an oxide skeleton and micropores). The number and size of the pores found in the final glass product (and, thus, the density of the final glass product) are a function of the rate of heating, the ultimate sintering temperature, and the period of time the xerogel is maintained at the ultimate sintering temperature. In sol-gel processes, an acid catalyst is generally used to speed the sol-gel reactions.
When used in applications relating to photoreceptor layers, sol-gel reaction components, and any desired additives, are mixed with conventional photoreceptor layer materials. The hydrolysis of the sol-gel reaction components takes place its situ in the coating solution. After coating, solvents used in the process evaporate, and a desired thin film forms. The condensation of the sol-gel reaction components takes place in situ during thermal drying, and an organic-inorganic interpenetrating network is formed, which unexpectedly provides better wear resistance, deletion control and other benefits.
However, these sol-gel formulations for preparing siloxane-containing materials, for example, can involve processes in which large volume changes may take place. Such issues make scaling sol-gel procedures beyond a laboratory scale difficult.
Thus, there remains a need for improved, scalable apparatuses and methods for preparing sol-gel solutions that will produce high yields of the desired sol-gel solutions having the electrical and physical properties on a large scale that are obtained on a laboratory scale.